1. Field
The following description relates to a quantum dot electroluminescent device and a method for fabricating a quantum dot electroluminescent device. In exemplary embodiments, the quantum dot electroluminescent device has a low turn-on voltage and a low operating voltage while exhibiting high brightness and high luminescence efficiency.
2. Description of the Related Art
A quantum dot is a crystalline semiconductor material having a size of about a few nanometers and consists of about several hundred to about several thousand atoms. The small size of a quantum dot results in a large surface area per unit volume. This allows most of the constituent atoms to be exposed to the surface, and gives rise to various effects, including quantum confinement. By taking advantage of the quantum confinement effect, the emission wavelength of the quantum dots may be controlled by varying the size of the quantum dots. Further, quantum dots have received a great deal of attention in the display arts for their advantages such as good color purity and high photoluminescence (“PL”) efficiency.
A quantum dot electroluminescent (“QD-EL”) device has a three-layer structure, as the basic structure, in which a quantum dot light-emitting layer is interposed between a hole transport layer (“HTL”) and an electron transport layer (“ETL”).
A conventional organic light-emitting diode (“OLED”) includes a HTL and a light-emitting layer. The hole transport layer (“HTL”) is formed of a material having a highest occupied molecular orbital (“HOMO”) level of between 5.0 and 5.3 eV. The light-emitting layer is formed of a material having a HOMO level of between 5.0 and 5.5 eV. The small difference in the HOMO levels between the light-emitting layer and the HTL allows for the efficient transport of holes through the layers to ensure high efficiency of the device.
However, a large band offset between the valence band level (about 6.8 eV) of the quantum dots used in the QD-EL device and the HOMO level of the HTL material used in the OLED may result in problems with the QD-EL device. This may cause problems relating to the turn-on voltage, the operating voltage, the overall efficiency, the carrier injection efficiency, etc. of the QD-EL device.
In an attempt to resolve these problems, a method has been considered for controlling the HOMO level of the HTL or for selecting a suitable HTL material that is capable of reducing the band offset between the HTL layer and the quantum dot (“QD”) layer. However, there are few known HTL materials that have a HOMO level as high as 5.4 eV. Further, the use of a HTL material having a HOMO level of 5.4 eV or above causes a large band offset between a HTL and an indium-tin oxide (“ITO”) anode, making the migration of holes throughout the layer difficult.
Under such circumstances, there is a need to develop a method that uses an HTL material that has excellent physical properties. There is a further need to develop a method that uses an HTL material that controls the band level of a QD light-emitting layer in order to lower the turn-on voltage and the operating voltage of a QD-EL device while improving the carrier injection efficiency of the QD-EL device.